you can set the %_topdir variable in the .rpmmacros configuration file (in your home directory) like this:

echo "%_topdir ~/foo/bar/rpmbuild" >> ~/.rpmmacros

It wouldn't (shouldn't) really make much difference; since sources are to be copied there by rpmbuild; then they are build and installed and packaged... Since all paths are supposed to be relative the outcome should be the same.

PS: more information on the rpm build area
PPS: a long time ago the default build area was in /usr/src/packages ; but then you had permission issues etc.

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